Digital storage devices such as a HDD-recorder or a DVD-recorder usually have an analog video input and an analog video output. Internally, however, the video signal is digitized for processing such as MPEG compression etc. An incoming analog video signal is first digitized by means of an 8, 9 or even 10-bit ADC (Analog-to-Digital converter), hereafter referred to as video converter, before any digital processing or compression is performed. The digitized video signal normally conforms to the ITU R-656 standard. The processed video signal may be stored on a recording medium such as for example a DVD disc, a VCD disc, or a DVHS tape. It may also be viewed directly after MPEG decoding. For this purpose the digitized video signal is converted back into an analog video signal by means of a DAC (Digital-to-Analog converter).
In this context U.S. Pat. No. 6,300,985 describes how to convert a normal analog CVBS signal into a Serial Digital Interface output signal.
Analog-to-digital conversion is performed using ICs that work well if there is an analog input video signal that is in conformity with a standard (ITU-R BT624 for PAL, SMPTE 170M for NTSC, etc.). Sometimes, however, the analog input video signal is not in complete conformity with a standard, e.g. when it comes from a VCR in trick mode (like pause or fast forward). In these cases e.g. the fields (upper field, lower field) are mixed up. Though the ADC IC still generates a digital video signal, this signal contains errors.
In the case when the analog input video signal conforms strictly to a standard such as NTSC and PAL, the encoded and decoded video signal displayed on the TV is normal. This means the interlaced field sequence is correctly displayed and there is no jitter of the picture.
However, when the analog input video signal does not conform strictly to the video standards, then the digitized video signal becomes defective, giving rise to imperfections such as a jittering picture when the 1st field and the 2nd field are displayed in the wrong sequence.